fix: Redact SSO PII before deletion

[ktyagiapphelix2u]

Description

Implements automatic PII redaction for UserSocialAuth records before deletion to prevent personally identifiable information from persisting in Snowflake soft-deleted records.

Problem

When users unlink SSO accounts or retire, the UserSocialAuth records are deleted from the LMS database but persist as soft-deleted records in Snowflake (via Fivetran) with the _fivetran_deleted flag. These soft-deleted records retained PII in the uid and extra_data fields, creating a data retention and privacy risk.

Solution

• Added redact_user_social_auth_pii() utility function to redact PII before deletion
• Implemented Django pre_delete signal handler to automatically redact PII for all deletion paths
• Updated retire_user management command to explicitly call redaction before deletion
• Redacted values: uid="redacted@retired.invalid", extra_data={}

Jira Ticket

https://2u-internal.atlassian.net/browse/BOMS-514